High-frequency tuning device



Jan. 7, G C A S N HIGH-FREQUENCY TUNING DEVICE Filed June 27, 1944 INVENTOR GlLBERT G. LARSON BY ATI' NEY Patented Jan 7, 1947 UNITED STATE s PATENT- OFFICE HIGH-FREQUENCY TUNING DEVICE Gilbert C.' Larson, Bayside, N. Y., assignor, by mesne assignments, to Hazeltine Research, Inc.',

Chicago, 111., a corporation of Illinois v Application June 27, 1944, Serial No. 542,374 '11 Claims. (Q1. 2:50-40) The present invention relates to high-frequency tuning devices and, particularly, to such reactance to resistance, and good frequency stability. Wave-signal transmisison lines oi! suitable length have resonant properties which render them desirable for this purpose and may be so constructed that they. exhibit the desirable characteristics last mentioned.

As is well known, a transmission line of nominal quarter-wave length and short-clrcuited at one end has a value of impedance measured at its open end which varies with its effective electrical length. In using such lines as resonant circuits, it is, of course, dmirable that the open end of the line shall always have maximum impedance at the operating frequency. In order that the effective electrical length of the line may be readily varied, thus to tune the line so that it has maximum impedance at any operating frequency in a range of such frequencies, it has been proposed thatan adjustable condenser be connected across the open end of the line to provide a high-frequency tuning device. The transmission line usually is of the concentric type closed at its ends by conductive discs which not only provide short circuits between the conductorsoi! the line at their ends, but additionally tatably supported to vary the area over which the condenser plates are in opposing relation.

All of these proposed arrangements last described have the disadvantage that they necessarily involve sliding electrical connections or contacts between the adjustable inner conductor portion and its support. The support of necessity is included iii the electrical circuit ofthe transmission line with the result that the sliding connections or contacts mentioned mustbe carefully designed and frequently serviced, thusincreasing the cost and complexity of the tuning There is the further disadvantage thateven the best designed sliding connections or" device.

contacts cause undesirable noise disturbances in a wave-signal apparatus using such tuning device and, equallyimportant, not only lower the ratio of the inductive reactanee to resistance,

or Q, of the tuning device, but additionally result in very erratic variations of its Q over the tuning range. ductive discs at the ends thereof provide for the transmission line what may essentially be considered a shielding housing. It is conventional to continue the adjustable portion of the inner conductor outside of this housing to provide a manually adjustable member by which to vary the spacing between the condenser plates. There is the additional disadvantage with this construction that the inner conductor when extended exteriorly of the housing'seldom is at ground potential and must be coupled to a manually adjustable tuning knob or the like by means of complicated electrical isolating devices in order that the frequency or stability of the tuning device shall not be disturbed by proximity of the hand during tuning. Since the portion of the inner conductor which extends exteriorly of the housing can be maintained atvground potential only with great dimculty, a serious problem is presented of preventing the undesirable transfer of wave-signal energy between such portion and other near-by wave-signal circuits. I

It is an object'of the present invention, therefore, to provide a new and improved highfrequency tuning device, adjustable over a predetermined frequency range, which avoids one or more of the disadvantages and limitations of prior tuning devices of thetype described. 3

It is a further object of the invention-to provide a new and improved high-frequency tuning device which is adjustable over a predetermined frequency range and which does not involve or require the use of sliding connections or contacts in the eflective electrical portion of its structure. I I

' It is an additional object of the invention to provide a new and improved high-frequency tuning device, adjustable over apredetermined frequency range, of simple and sturdy construc- -tion and one which has high frequency stability The outer conductor and the 0011- I and a relatively permanent frequency calibration.

It is an additional object of the invention to provide an improved high-frequency tuning device, adjustable over a predetermined frequency range, which not. only has a high ratio of inductive reactance to resistance but one inwhich this ratio varies only with the resonant frequency of the device over the adjustable range.

It is a further object of the invention to provide an improved high-frequency tuning device, adjustable over a predetermined frequency range, which is adapted to be enclosed within a conductive shielding housing and one in which a conductive tuning shaft extending exteriorly of the housing is at ground potential both within and without the housing and thus is not subject to undesirable capacitive hand effects during a tuning adjustment or to undesirable coupling of wave-signal energy to adjacent wave-signal circuits. I

In accordance with the invention, a high-frequency tuning device adjustableover a predetermined range of frequencies comprises a transmission line effectively short circuited at both ends and inclu ling a pair of parallel conductors having an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in the range, a pair of stationary condenser p'lates individually connected to the conductors substantially at the effective mid-points thereof, and a movable condenser plate adapted to move into relatively closely spaced relation-with both of the stationary plates to provide a balanced condenser structure adjustable to tune the device over the frequency range, whereby the device has points of minimum wave-signal potential which maintain fixed positions on the tuning device over the range of adjustment thereof.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following descrip tion taken in connection with the accompanying drawing, and its scope will be pointed .out in the appended claims.

Referring now to the drawing, Fig; 1 represents a cross-sectional elevational view of a high-frequency tuning device embodying the present invention; and Fig. 2 illustrates a plan view, partially broken away and partially cross-sectioned to show constructional details of the tuning device of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, there is represented a high-frequency tuning device embodying the present invention in a particular form. The tuning device way between the conductors and normal to a plane which includesthe conductor axes, whereby the transmission line I5 is positioned symmetrically within the housing III. The conductors I3, I4 have an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in the range over which the tuning device is adjustable. A conductor length of one half-wave length is usually atisfactory for most applications. These conductors preferably have such diameter that the ratio of their diameter to the conductor spacing is approximately equal to to provide a substantially maximum ratio of inductive reactance to resistance, or maximum Q. In therelationship last expressed, it is the surface area and thus the circumferenceof the conductors which largely affects the maximum Q of the tuning device. Since the circumference is equal to 1r times the conductor diameter, the relationship mentioned may be expressed in another manner; namely, that substantially maximum Q is attained when the ratio of the circumference of either conductor to the spacing therebetween is equal to 1/23. The last-mentioned ratio is perhaps most useful when the conductors I3 and I4 are of square or rectangular cross sections. The tuning device also includes at least one pair of stationary coplanar condenser plates I 8, I1 individually connected to the respective conductors I3, I4 substantially at the effective mid-points of the latter, at least one movable condenser-plate I8 adapted to move into relatively closely spaced overlapping relation with both of the stationary widely spaced to minimize the capacitance be tween them and the connection of each of these plates to an individual one of the conductors I3,

II is preferably made at a plurality of points, as by spaced terminal brackets 20, 2I and 22, 23, symmetrical about the mid-point of the conductor. This multiple connection of the condenser plates to the conductors minimizes the inductive reactance of the circuit between each conductor and its associated condenser plate. The stationary and movable plates of the condenser thus provided preferably have such configuration as to provide a condenser structure of the butterfly type, which derives its name from the similarity. of its condenser plate shape to that of a butterfiys wing. further to minimize the capacitance of the condenser when the movable plate-is adjusted to its minimum capacitance position indicated inFig. 1.-

- The tuning device is provided with means for coupling the transmission line I5 to an external wave-signal circuit, not shown, this means comprising a coupling loop 24 supported by the housing I8 in inductively coupled relation to the transmission line I5 adjacent a short-circuited end thereof. In particular, the loop 24 extends through an insulating bushing 25 secured in an aperture in the housing I 8 and the end of the loop is electrically and mechanically connected to the housing. A similar loop 26 may be positioned at the other end of the transmission line I5, if desired, to couple the transmission line to the same or another wave-signal circuit, not

shown.

The manner in which the conductors of the transmission line I5 are connected to the end walls of the housing I8 is more clearly evident from the plan view of Fig. 2. The conductors are preferably hollow and have a shoulderedplug 2'I securely fitted into each end thereof. Each of the plugs 21 extends through an aperture 28 provided in the end walls II and I2 and have a screw-threaded end upon whichis threaded a securing nut 28.

Fig. 2 also shows more clearly the constructional details of the condenser and it will be seen that a plurality of stationary and movable plates I8, I! and I8 are used. The stationary plates it are supported from spaced insulator end members 30, 3| by longitudinal conductive rods 32, 33 which are electrically connected to the condenser terminal brackets 20, 2|, respectively. The stationary plates I'I are similarly supported. The terminal brackets 20-23, inclusive, mechanically support the insulator member 30, from the conductors l3, l4 and thereby contribute to the rigidityv of the tuning device. The insulator member 3| is referably supported from one wall of the housing i0 further to enhance the ri idity of the structure. The shaft I9 for the movable condenser plates 89 is journalled in the insulator members 30, 3! in conventional manner-and extends through an aperture 34 of the housing 10 to enable adjustment of the condenser from a point outside the housing.

Considering now the operation of the tuning device just described, wave-signal energy from an external wave-signal circuit, not shown, is coupled through one of the loops 24 or 26 into the transmission line IE to set up therein a standing wave of current having maximum amplitudes at the ends of the transmission line and a minimum amplitude near its center. potential is also set up along the transmission line 85 and has minimum amplitudes at the ends of the transmission line and a maximum amplitude near the center 'of the line. The condenser, comprising the stationary plates it, ii and the movable plate it, capacitively loads .the center of the transmission line it and thus determines the efiective electrical length of the line. The standing wave of voltage and current along the line have maximum amplitudes when the effective electrical length of the latter is an odd number of half-wave lengths at the freqency of the wave-signal energy applied thereto. This condition is established by adjustment of the movable plate it of the condenser. Wavesignal energy may be applied from the transmission line IE to a second external wave-signal sircuit, not shown, by coupling the latter to the other of the loops M or 2t.

It will be noted that the condenser shaft 2s lies in a plane normal to the plane which includes the axes of the conductors it, it and that the plane which includes the condenser shaft is midway between the axesoi the conductors. The shaft thus is in a neutral plane, or plane of minimum electric-field intensity, and introduces minimum loss into the transmission line I5. Additionally, it will be seen that the electrical paths over which wave-signal 'currents flow in the transmission line l5 do not include any sliding connection or contacts. This ensures that the ratio'of-the inductive reactance to resistance of the tuning device is not subjectto erratic variations with adjustment of the condenser, but rather varies only with the resonant frequency of thetuning device.

In certain instances, a secondary resonantcircuit condition may be established which impairs the operation of the tuner circuit. When this condition obtains, a second resonant circuit may be traced from the stationary condenser plates i8 and I1, through the conductors l3 and I4 considered in parallel. through the end-wall members II, I! or the housing l0; and through the condenser shaft to the movable condenser plates l8. Such secondary-resonance eflects A standing wave oi may be substantially eliminated by including a suitable suppression resistor or approximately 100 ohms between the condenser shaft I9 and the housing iii in the vicinity of the aperture 34. This resistor is connected to the condenser shaft l9 through a suitable sliding contact. Although this involves a sliding contact in the tuner construction, the sliding contact has no deleterious effect since it cannot be construed as part of the the tuner circuitin that substantially no wavesignal current of the resonant frequency passes through this contact.

While the tuner hasb'een described as havin inductive-couplingloops 24 and 28 by which the tuner is coupled to one or more external wavesignal circuits, not shown, it will be apparent that such coupling may also be provided by one or more capacitive probes extending through a wall of the housing iii in insulated relation therewith into proximity with one of the onductors i3 or M. The size of a capacitive probe of this nature and its spacing from the conductor is dependent in large part upon the impedance of the external wave-signal circuit, not shown, and the degree; or

- magnitude of coupling desired between the tuner and the external circuit.

A tuning device embodying the present invention is of simple and rigid construction, has relatively high frequency stability, and is characterized by a permanent frequency calibration. Additionally, the condenser shaft it, as previously explained, extends throughout its length in a region of minimum magnetic-field intensity and in a substantially neutral or zero electrical field. It will therefore be apparent that the condenser shaft is at ground potential throughout its length and thus is free from any tendency to couple wave-signal energy to adjacent wave-signal circuits located externally oi the housing it. This fact further ensures a high degree of freedom from any tendency oi the frequency or stability of the tuning device being upset when the adjusting shaft it of the condenser is manually grasped to eliect a tuning ustment.

While there hasbeen described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

A high-frequency tuning device adjustable over a predetermined range of frequencies comprising. a transmission line effectively shortcircuited at both ends and including a pair or parallel conductors having an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in said range, at least one'pair of stationary condenser plates relatively widely spaced and individually connected to respective ones of said conductors substantially at the efiective mid-points thereof, and at least one movable condenser plate adapted to move into relativel closely spaced relation with said stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof.

2. A high-frequency tuning device adjustable 7 over a predetermined range of frequencies comprising, a transmission line effectively short-circuited at both ends and including a pair of parallel conductors having an effective electrical length equal approximately to an odd number of half-wavelengths at a frequency in said range,

at least one pair of coplanar stationary condenser plates relatively widely spaced and individually connected to respective ones of said conductors substantially at the effective midpoints thereof, and at least one movable condenser plate adapted to move into relatively closely spaced overlapping relation with said stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which connected to respective ones of said conductors substantially at the effective mid-points thereof, and at least one movable condenser plate.adapted to move into relatively closely spaced relation with said stationary plates to provide a'balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof.

4. A high-frequency tuning device adjustable over a predetermined range of frequencies comprising, a transmission line effectively shortcircuited at both ends and including a pair of parallel conductors having an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in said range, said conductors having a ratio of the transverse cross-sectional circumference thereof to the conductor spacing'su'bstantially equal to 1r/2.7 to

provide for said line a substantially maximum .ratio of inductive reactance to resistance, at least prising. a transmission line effectively short-circuited at both ends and including a pair of paral lel conductors having an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in said range, at least one pair of stationary condenser plates relatively widely spaced and individually connected to respective ones of said conductors substantially at the effective mid-points thereof, at least one movable condenser plate adapted to move into relatively closely spaced relation with 8 said stationary plates-to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof, and an adjusting shaft connected to said movable condenser plate and positioned throughout its length in a region of minimum magnetic-field intensity of said line.

6. A high-frequency tuning device adjustable over a predetermined range of frequencies comprising, a transmission line effectively short-circuited at both ends and including a pair of parallel conductors having an efiective electrical length equal approximately to an odd number of half-wave lengths at a frequency in said range, at least one pair of stationary condenser I condenser structure adjustable to tune said deplates relatively widely spaced and individually connected to respective ones of said conductors, the connection of each of said plates to an individual conductor being made at a plurality of points symmetrical about the effective mid-point of said individual conductor, and at least one movable condenser plate adapted to move into relatively closely spaced relation with said stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof.

7. A high-frequency tuning device adjustable over a predetermined range of frequencies comprising, a transmission line eifectively short-circuited at both ends and including a pair of parallel conductors having an effective electrical length equal approximately to an odd number of half-wave lengths at a frequency in said range, at least one pair of stationary condenser plates relatively widely spaced and individually con nected to respective ones of said conductors substantially at the effective mid-points thereof, and

at least one movable condenser plate adapted to move into relatively closely spaced relation with said stationary plates to provide a balanced vice over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof, said stationary and movable plates having such configuration 'as to provide a butterfly type of condenser structure to minimize the capacitance of said-condenser when said movable plate is adjusted to its minimum capacitance position.

8, A high-frequency tuning device adjustable over a predetermined range of frequencies comprising, a conductive housing havingv opposing conductive wall members, a pair of parallel conductors electrically and mechanically connected to said wall members to provide within said housing a transmission line short-circuited at both ends and having an effective electrical length approximately one half-wave length long at a frequency.in said range, at least one pair of stationary condenser plates relatively widely spaced and individuallyconnected to respective ones of said conductors substantially at the effective mid-points thereof, and at least one movable condenser plate adapted to move into relatively closely spaced relation with said stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on sad tuning device over the range of adJustment thereof.

9. A-high-frequency tuning device adjustable overa predetermined range of frequencies comprising, a conductive housing having opposing conductive wall members, a pair of parallel conductors electrically and mechanically connected to said wall members to provide within said housing a transmission line short-circuited at both ends and having an effective electrical length approximately one half-wave length long at a frequencyin said range, the points at which said conductors are connected to said wall members being symmetrical about a plane lying midway between said conductors andnormal to the plane which includes the conductor axes thereby to position said transmission line symmetrically within said housing, at least one paid of stationary condenser plates relatively widely spaced and individually connected to respective ones of said conductors substantially at the effective mid-points thereof, and at least one movable condenser plate adapted to move into relatively closely spaced relation with said stationary plates to provide a balanced condenser structure adjustable. to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof.

10. A high-frequency tuning device adjustable over a predetermined range of frequencies comprising, a conductive housing having opposing conductive wall members, a pair of parallel conductors electrically and mechanically connected to said wall members to provide within said housing a transmission line short-circuited at both ends and having an eflective electrical length approximately one half-wave length long at a frequency in said range, at least one pair of stationary condenser plates relatively widely spaced and individually connected to respective ones of said condudctors substantially at the effective mid-points thereof. at least one movable condenser plate adapted to move into relatively closely spaced relation with said stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency said tuning device has points of range, whereby minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof, and means for coupling said line to an external wave-signal circuit.

11. A high-frequency tuning device adjustable over a predetermined range of frequencies comprising. a conductive housing having opposing conductive wall members, a pair of parallel conductors electrically and mechanically connected to said wall members to provide within said housing a transmission line short-circuited at both ends and having an effective electrical length approximately one half-wave length long at a frequency in said range, at least one pair of stationary condenser plates relatively widely spaced and individually connected to respective ones of said conductors substantially at the effective mid-points thereof, at least one movable condenser plate adapted to move into relatively closely spaced-relation withsaid stationary plates to provide a balanced condenser structure adjustable to tune said device over said frequency range, whereby said tuning device has points of minimum wave-signal potential which maintain fixed positions on said tuning device over the range of adjustment thereof, and a coupling loop supported by said housing and inductively coupled to said line adjacent a short-circuited end thereof for'coupling said line to an external wave-signal circuit.

- GILBERT c.14aRsoN. 

